If a mobile communication system operates in the manner that increases the signal efficiency of a frequency band, such as in Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), Worldwide Interoperability for Microwave Access (WiMAX), or Long Term Evolution (LTE), it should use a complex modulated signal in order to increase signal efficiency in a given frequency band. This complex modulated signal is a non-constant envelope signal with a relatively high Peak-to-Average Power Ratio (PAPR). Linearity is a very critical factor to the non-constant envelope signal. Accordingly, the mobile communication system should amplify a signal, while maintaining high linearity and high efficiency.
Although the high linearity is maintained by use of a low-efficiency power amplification apparatus, heat is generated due to the low efficiency. Therefore, the mobile communication system should be equipped with a relatively large, unnecessary cooling system to reduce the heat. However, the cooling system brings an increase in size and cost to each of a Base Station (BS), a repeater, and a Mobile Station (MS) in the mobile communication system. In this context, active research is conducted on a high-efficiency power amplification apparatus that maintains high linearity and high efficiency for the CDMA, WCDMA, WiMAX, or LTE mobile communication system.
Major high-efficiency power amplification apparatuses include Class-E, inverse Class-E, Class-D, and inverse Class-D ones. These power amplification apparatuses commonly achieve high efficiency through switching of a power device.
However, the Class-E, inverse Class-E, Class-D, and inverse Class-D power amplification apparatuses are not capable of zero-voltage switching because of parasitic harmonics and the large output capacitance of a high-output power device. As a consequence, it is difficult to implement the power amplification apparatuses with packaged power devices.
Other major high-efficiency power amplification apparatuses are Class-F and inverse Class-F ones. The Class-F and inverse Class-F power amplification apparatuses offer high efficiency through harmonic control. Yet, the high efficiency is not achieved easily because of the difficulty in the harmonic control of a current source of a power device simultaneous with compensation of the output capacitance of the power device. Moreover, a complex harmonic control circuit and a fundamental frequency matching circuit are essential for real implementation of the Class-F and inverse Class-F power amplification apparatuses.
As described above, real implementation of high-efficiency power amplification apparatuses proposed to date is difficult because an additional circuit is needed, for example, a harmonic control circuit is needed for the Class-F and inverse Class-F power amplification apparatuses.
Accordingly, there exists a need for a high-efficiency power amplification apparatus for achieving high efficiency without an additional circuit.